A grease trap is a collection device connected into the drain lines between the kitchen sinks and the outflow lines to the sewer system.
Its function is to reduce the flow rate of grease laden water, delaying it within a confined chamber, thus allowing the grease to rise or float to the surface and be trapped in the liquid surface of the chamber while the solids settle to the lower level of the trap. The outflow of the grease trap must be connected to a sanitary or combined sewer.

Flow from undersink grease traps or directly from plumbing fixtures enters the grease interceptor. The UPC requires that all flow entering the interceptor must enter through the inlet pipe.

An approved flow control or restricting device is installed to restrict the flow to the grease interceptor to the rated capacity of the interceptor.

An air intake valve allows air into the open space of the grease interceptor to prevent siphonage and back-pressure.

Oil and grease floats on the water surface and accumulates behind the grease retaining fittings and the wall separating the compartments. The oil and grease will be removed during routine grease interceptor cleaning.

Solids in the wastewater that do not float will be deposited on the bottom of the grease interceptor and will need to be removed during routine grease interceptor cleaning.

Grease retaining fittings extend down into the water to within 12 inches of the bottom of the interceptor. Because grease floats, it generally does not enter the fitting and is not carried into the next compartment. The fittings also extend above the water surface to provide air relief.

Some grease trap interceptors have a sample box so that inspectors or employees of the establishment can periodically take effluent samples. Having a sample box is recommended by the UPC but not required.

Flow exits the interceptor through the outlet pipe and continues on to the sanitary sewer system.